The present invention relates to methods and means for strengthening titanium alloys for operation at higher temperatures. More particularly the invention relates to a method and means by which a titanium alloy is rendered capable of operating with good physical properties at temperatures above those at which the metal normally loses or otherwise loses its good operating physical properties.
At the present time titanium and its alloys can be used at temperatures up to about 1100.degree. F. It is a superior metal exhibiting a good set of properties and many uses have been made of it for applications at temperatures up to about 1100.degree. F. If titanium could be modified so that its effective operating temperatures were above about 1100.degree. F. it could be employed in the place of more expensive superalloys which are presently used in applications requiring the combination of high strength at high temperature. The superalloys are employed in the temperature range of over 1000.degree. F. up to about 1700.degree. F. Many applications exist for a metal having good strength and other properties in the temperature range of 1100.degree., to 1300.degree. F. and if such a titanium alloy existed it could be substituted for more expensive superalloys presently employed in applications which require high strength at these temperatures.
The high reactivity of titanium and titanium alloys is manifested in its dissolution of most carbides, oxides, and other refractory compounds generally thought to have high chemical stability in other alloy systems. There has been much work to find ceramic compounds which resist dissolution by liquid titanium. All studies have concluded that every material which has been examined reacts with liquid titanium.
Recently, it has been found that rare earth additions to titanium produce stable sesquioxide compounds that have stability in the solid state in titanium alloys and which dissolve in the liquid state. This has been extended with rapid solidification processing by Sastry and co-workers to yield a fine dispersoid of rare earth-based particles. This work is reported in an article by Sastry et al, entitled "Structure and Properties of Rapidly Solidified Dispersion Strengthened Titanium Alloys: Part 1. Characterization of Dispersoid Distribution, Structure and Chemistry" Met. Trans. A. Vol. 15A, pp. 1451-1463, 1984. Sastry et al have demonstrated the stability of these dispersoid particles to high temperatures. See in this regard SML Sastry et al "Dispersion Strengthened Powder Metallurgy Titanium Alloys" Final Report, Contract No. F33615-81-C-5011, Report AFWAL-TR-83-4092, October 1983.
It has been known heretofore that high temperature strengthening of conventional titanium alloys has been accomplished through solid solution strengthening techniques. This is brought out in the article by H. K. Miska appearing on pages 79 and 80 of the July 1974 issue of Materials Engineering under the title "Titanium and Its Alloys". There are several reasons why solid solution strengthening techniques have been employed. One reason is the high chemical stability of titanium solid solutions. These solutions exist as stable phases up to transformation temperatures on the order of 1700.degree. F.
Solid solution strengthening has an apparent upper temperature limit for effective strengthening. Examination of the properties of current titanium alloys would lead one to the conclusion that the limit of titanium alloy strengthening is a result of this limitation of solution strengthening. One way of providing an increment of strengthening beyond solution strengthening is to add a precipitate or dispersoid phase to a solid solution strengthened alloy. It is known that the temperature dependence of dispersion strengthening is a weak function of temperature, varying only as the temperature dependence of elastic moduli. Dispersoid strengthening continues to be effective at temperatures beyond the temperatures at which solution strengthening is effective. Prior art examples of dispersion strengthened alloys in alloy systems other than titanium are the thoria-dispersion strengthened nickel alloys, and dispersion strengthened alloys produced by mechanical alloying. In all cases, the dispersoid phase is stable to temperatures far above the limit of solution or precipitation strengthened alloys. Unfortunately, for titanium alloys, the stability of most precipitate phases has been inadequate to prevent the dissolution or coarsening of the precipitate and aging of the material during high temperature service. This has been reported for several candidate precipitation strengthening systems by K. C. Anthony in a report AFML-TR-67-352 in November 1967 under the title "Dispersion Strengthened Alpha Titanium Alloys".
Further, the second phase compounds which could potentially serve as stable precipitate compounds are found to exhibit strong segregation during solidification thus limiting their usefulness as strengthening agents. Also the segregation results in the precipitation of large, blocky precipitate which not only do not contribute to strengthening but tend to weaken the alloy by providing sources of early crack nucleation. This is brought out in the report of K. C. Anthony above.
Furthermore, precipitation strengtheners such as Ti.sub.3 Al exhibit slip localization due to the limited slip systems available in Ti.sub.3 Al. Slip localization leads to a low potential for work hardening and also leads to low ductility failures in alloys containing Ti.sub.3 Al. Ti.sub.3 Al also exhibits inadequate thermal stability to prevent precipitation or re-precipitation during high temperature service of the alloy. The phenomenon of post creep embrittlement observed in many high temperature titanium alloys is attributed to the precipitation of Ti.sub.3 Al along slip bands following creep exposure. Embrittlement of titanium alloys containing Ti.sub.3 Al is thus due to the limited thermal stability of Ti.sub.3 Al.
It has been observed by Rath et al. in their publication "Influence of Erbium and Yttrium on the Microstructures and Mechanical Properties of Titanium Alloys", B. B. Rath, B. A. MacDonald, S. M. Sastry, R. J. Lederick, J. E. O'Neal, and C. R. Whitsett, in Titanium '80, Editors H. Kimura and O. Izumi, Proc. Fourth Int'l. Conf. on Titanium, Kyoto, Japan, May 1980, Met Soc AIME, 1980 that the addition of rare earth metals, typically Er, Gd, Y and others, produce a dispersoid which is stable to temperatures as high as 1472.degree. F. Sastry further observed that rapid solidification of these alloys from the melt prevented gross segregation of these alloying elements, and formed a fine, uniform distribution of dispersoids.
Further, Sastry has reported a significant beneficial strengthening effect due to the formation of this fine rare earth dispersoid. It is believed that a fine dispersoid is required to achieve a small spacing between particles on any plane in the alloy in order to achieve significant strengthening. It is also believed that a beneficial strengthening effect drops off rapidly as the size of the particles increases by the process of solid state particle coarsening. It is for this reason that the thermal stability of the dispersoid must be such that particle coarsening is minimized during high temperature exposure.
The rare earth sesquioxides, Er.sub.2 O.sub.3 and Y.sub.2 O.sub.3 have been identified to be the stable dispersoid phases in titanium alloys containing Er or Y, respectively. The above article by Sastry reports coarsening of dispersoids formed by the addition of the rare-earth elements Er, Nd, Dy, Gd, Y, and Ce in titanium containing impurity oxygen in an amount sufficient to create rare earth oxide compounds. It has been found that the alloy Ti-Er produces the dispersoid with the greatest resistance to coarsening at high temperatures. This alloy exhibited only modest coarsening at 800.degree. C., but in 45 minutes at 900.degree. C., particle coarsening was observed from a mean particle diameter of 450A to a mean diameter of 680A. The loss in dispersoid strengthening resulting from this coarsening amounts to approximately 20%. I have examined a titanium base alloy with a composition as follows:
Ti-6W/o, Al;2w/o, Sn;4w/o, Zr;2w/o, Mo;1w/o, Er;0.25w/oB, and the balance titanium.
In this composition description w/o stands for weight percent. This composition also contained Er.sub.2 O.sub.3 dispersoid particles dispersed therein. The composition exhibited particle coarsening after a one hour anneal at 950.degree. C. This particle coarsening was accompanied by an increased mean interparticle spacing from 0.67 micrometer to 1.87 micrometer. The reduction in strengthening due to the Orowan-Ashby model was 46%. Because of the need to minimize particle coarsening, Sastry reported that his consolidation techniques were carried out at 820.degree.-850.degree. C.
These observations point out the critical importance that dispersoid particle stability makes in the ability to process alloys at elevated temperatures and in the ability of the alloy to withstand long term exposure at somewhat lower temperatures.
The thermodynamic stability of rare earth oxides is great enough so that the equilibrium vapor pressure of oxygen in equilibrium with them at temperature up to the melting point of titanium is so small that in an ideal solid solution, the concentration of oxygen in solution in the alloy required to stabilize the oxide is so small it would be unmeasurable by conventional analytical techniques. This stability is the basis for the use of oxide ceramics for the containment of most liquid metals. Titanium is far from an ideal solid solution, however, and the free energy of solution of oxygen in titanium solid solution is on the order of -125 Kcal/mole at low oxygen concentrations. For an ideal solid solution, the excess free energy of solution is zero. The standard free energy of formation of the oxide compound, TiO.sub.2, is -112 Kcal/mole. Subtracting the excess free energy of solution of oxygen in titanium (-97 Kcal/mole), one can see that the net free energy of formation of the reaction of TiO.sub.2 with titanium solution of oxygen at low concentrations is very small. Solid solution of oxygen in titanium is hence more energetically favored than the oxide for oxygen concentrations below a few percent oxygen.
Although the heat of solution of oxygen in titanium reduces the negative free energy of reaction of most rare earth oxides in equilibrium with titanium, most rare-earth oxides are energetically favored at some low oxygen content of the solid solution. This is verified by the experimental results of Sastry et al, who have established the practical stability of Er.sub.2 O.sub.3 and other rare earth oxide compounds to temperatures as high as 850.degree. C.
Rare earth element additions have been shown to produce fine precipitate dispersions based upon the insolubility of rare earths such as erbium and terbium in titanium. Rapid solidification processing has been shown to be required to produce this dispersion. This information was presented at the poster session of the National Bureau of Standards Conference on Rapid Solidification technology, December of 1982 and in written reports by S. Sastry. The stability of the dispersoids produced by these rare earth additions appears adequate for short-term exposure to temperatures in the neighbor range of 800.degree. C. but this temperature may not be adequate to allow high temperature consolidation of these alloys or may limit their maximum surface temperature.
Sulfur is generally avoided as a tramp element in titanium alloys. This is because of the potential for the formation of titanium sulfides which would lead to embrittlement in service. I have found that in the presence of rare earth elements in solution, the equilibrium concentration of sulfur may be maintained below the level for precipitation of Ti.sub.5 S or other titanium sulfides. It is known that manganese can be added to steels to eliminate the embrittling effect of residual sulfur by the formation of MnS. D. F. Stein, "Reversible Temper Embrittlement", Ann. Rev. Mater. Sci., Vol. 7, pp. 123-153, 1977.
I have examined dispersoid compounds in systems other than pure oxide compounds in order to identify a dispersoid compound which has greater thermal stability and hence a greater resistance to high temperature exposure than these sesquioxide compounds. It is well known that in equilibrium with their own vapor pressure, no compounds are known which have greater high temperature thermodynamic stability than the oxides of the rare earths. Their stability in equilibrium with solid titanium depends also on the excess free energies of solution of oxygen and the rare earth element with a titanium solid solution. Because of this, chemical compounds which have a lower free energy of formation in equilibrium with their own vapor pressure could have greater stability in equilibrium with a titanium solid solution. This could occur if the heat of solution of its constituent elements were less than that of the rare earth oxides. The compounds which have their own thermodynamic stability second only to the rare earth oxides are the rare earth oxysulfides.
I have observed that cerium sulfides and oxysulfides produce a fine dispersoid when added to titanium alloys and rapidly solidified from the melt.
I have now observed that rapid solidification technology applied to alloys containing cerium and sulfur additions offers a new opportunity for titanium alloy development. Pursuant to this I have found that by rapidly solidifying an alloy from the liquid state these new dispersion strengthening elements may be introduced into the alloy without the problem of coarse segregation zones of precipitation as has been observed in the prior art as discussed above. I find that dispersion strengthening compounds must have the characteristic that during or after solidification a fine particle dispersion is produced and in addition I have observed that such fine particle dispersion must be thermodynamically or kinetically stable.
Further, I have observed that sulfides and oxysulfides of certain rare earth element additions are found to produce fine precipitate dispersions based upon the insolubility of the sulfides and oxysulfides of rare earths such as cerium in titanium. It has now been observed that rapid solidification processing is required to produce this dispersion.
The stability of the dispersoids produced by these rare earth sulfide and oxysulfide additions appears adequate for short term exposure to temperatures in the range of 950.degree. to 1000.degree. C. This is at least 100.degree. C. higher than the capabilities of dispersion strengthened alloys containing only rare earth additions which produce dispersoids of the sesquioxide type compounds.